With the development of the air bag as a safety device for automobiles and other passenger vehicles, there has developed a need for a crash-sensing device for actuating the air bag inflater in a crash situation. This requires a detection device mounted on the vehicle for sensing a rapid change of velocity of the vehicle and actuating a switch when the deceleration is greater than a threshold amount. To be most effective, a crash-sensing device is preferably mounted at the front of a vehicle, such as on the front bumper where the change in velocity is most abrupt and acts with the minimum of time delay following the onset of a crash. At such locations, however, the device is exposed to other forces not connected with a crash situation but which may still be relatively large in magnitude. Thus the device must be direction sensitive, must be extremely rugged in construction, and must be able to discriminate both against high accelerations of very short duration to which the front of the vehicle is normally subjected, and discriminate against large velocity changes which nevertheless take place over a relatively long period of time, such as are experienced in emergency braking of the vehicle.
Crash-sensing devices using an inertial mass are known in the art. See, for example, U.S. Pat. Nos. 3,556,556 and 3,750,100. Inertial switches of the same general type have also been proposed which utilize the movement of an inertial mass under an acceleration or deceleration force. Such known devices have been used to sense acceleration but also, by means of fluid damping, have been used as velocimeters to respond to the integral of the acceleration. Fluid damping has been provided by enclosing the inertial mass in a closed chamber, the inertial mass acting as a piston dividing the chamber into two volumes. Any force acting on the piston is damped by the transfer of fluid from the decreasing volume side of the moving piston to the increasing volume side of the piston, as through a space around the piston or through a tubular passage between the two volumes. See U.S. Pat. Nos. 3,632,920 and 3,300,603.
In such piston-damped devices, the damping force as a function of velocity can be controlled by the nature of fluid flow passing through an orifice from the compression side to the vacuum side of the moving piston. Such conventional damped acceleration switches require a very high manufacturing tolerance to achieve the characteristics necessary to make them effective as crash-sensing devices. Such piston devices have also exhibited poor reliability and inconsistent performance with changes in temperature.